Yeah really its based on long-term t-bills and the risk premium. Because safe interest rates went up, people can get a better deal on long term debt unless the

Yeah really its based on long-term t-bills and the risk premium. Because safe interest rates went up, people can get a better deal on long term debt unless they get a discount on the bonds from SVB so that in the end the SVB bonds work out to a the current interest rates, if held to maturity. This is how bond prices move opposite to the external interest rate environment. There is also a risk premium since t-bills are considered perfectly safe, while these mortgage backed securities are likely now carrying a risk premium so buyers want an even higher effective interest rate out of them. It isn't really new-vs-old, it is just the interest rate environment. If there was zero new issuance of debt people would still be trading the old debt at the same interest rates and the value of the bonds held by SVB still would have been underwater. Of course if there wasn't enough debt to satisfy the appetite for buying debt then the prices would rise and get bid up, but we'd see this as a lowering of interest rates in these markets (which isn't happening).